Friday, October 10, 2014

In the last post, I talked about the design of the TransformComponent. Today we will look at how we can store entities as resources.

Dynamic and Static Data

I’m a huge fan of compiling resources into big blobs of binary data that can be read directly into memory and used as-is without any need for “deserialization” or “reference patching”.

This requires two things:

First, the data must be swapped to the right endianness when the data is generated.

Second, internal references in the resource must use offsets instead of pointers, since we don’t know where the loaded resource will end up in memory and we want to avoid pointer patching.

Initially, this approach can seem a bit complicated. But it is actually a lot simpler than messing around with deserialization and reference patching.

Note though, that this approach only works for static (read-only) data, such as meshes, textures, etc. If data needs to change for each instance of a resource, we must store it somewhere else. If an instance needs to change color, we can’t store that color value in a memory area that is shared with other instances.

So what typically happens is that we split out the dynamic data as “instance data” and let that data refer to the static resource data. Many instances can make use of the same resource data, thus saving memory:

We typically hard-code what goes into the instance. For example, we know that the color is something that we want to modify so we add it to the instance data. The vertex and index buffers cannot be changed and thus go into the static data. When we create the instance we initialize the instance data with “default” values from an instance template in the resource data.

You could imagine doing this another way. Instead of hard-coding the data that can be modified per instance, you could say that everything can be modified per instance. In the instance data you would then use a flexible key-value store to store the delta between the instance and the resource data.

This is more flexible than hard-coding, because it allows you to override everything per instance, even texture or vertex data if you want that. It can also save memory, because the instance data will only contain the things you actually override, not everything that potentially could be overridden. So if you have many instances that use the default values, you don’t have to store any data for them.

On the other hand, there are drawbacks to this approach. Accessing value becomes a lot more complicated and expensive, because we always need to perform an extra query to find out if the instance has overridden the default value or not.

We currently don’t use this approach anywhere in the engine. But I think there are circumstances where it would make sense.

Anyway, I’m getting sidetracked. Back to the entity system.

The thing is, the way I envision the entity system, it is very dynamic. Components can be added and removed at runtime, child entities linked in and properties changed. Components that handle static data, such as the MeshComponent do it by referencing a separate MeshResource that contains the mesh data. There is no mesh data stored in the component itself.

Since everything in the entity system is dynamic, there is only instance data. The only thing we have in the resource is the template for the instance data. Essentially, just a set of “instructions” for setting up an instance. There is no need for the instance to refer back to the resource after those instructions have been followed.

Defining the Resource Format

So an entity resource should contain “instructions” for setting up an entity. What should it look like? Let’s start by just writing up what needs to go in there:

Note: Of course the above is not legal C++ code. I’m using some kind of C-like pseudo-code that allows things like dynamically sized structs in order to describe the data layout. I’ve written about the need for a language to describe data layouts before.

The exact binary layout of the ComponentData is defined by each component type, but let’s use a common wrapper format:

Now we have a common way of identifying the component type, so we know if we should create a MeshComponent, a TransformComponent or something else. We also know the size of the component data, so if we should encounter a component type that we don’t understand, we can ignore it and skip over its data to get to the next component. (Another option would be to treat unknown component types as a fatal error.)

A quick fix to make this layout slightly better is to move all the fixed size fields to the start of the struct:

Now we can access the num_children parameter without having to look at all the components and their sizes to know how far we need to skip forward in the resource to get to the num_children field.

This may or may not matter in practice. Perhaps, we only need the value of num_children after we have processed all the component data, and at that point we already have a pointer into the resource that points to the right place. But I always put the fixed size data first as a force of habit, in case we might need it.

Sometimes, it makes sense to add offset tables to these kinds of resources, so that we can quickly lookup the offset of a particular component or child, without having to walk all of the memory and count up the sizes:

The first time you encounter code like this it can seriously spin your head around with all the casting and pointer arithmetic. However, if you think about what happens and how the data is laid out in memory it is really pretty straight forward. Any mistakes you do will most likely cause huge crashes that are easy to find, not sneaky subtle bugs. And after a while you get used to these kinds of manipulations.

But, anyway, I’m drifting off on a tangent again, because actually for our purposes we don’t need these lookup tables. We will just walk the memory from beginning to end, creating one component at a time. Since we don’t need to jump around between different components, we don’t need the lookup tables.

What we do need though is some way of storing more than one resource. Storing one entity is fine if we are dealing with a “prefab” type of resource, that contains a definition of a single entity. However, what about a level? It will probably contain a bunch of entities. So it would be nice to have a resource type that could store all those entities.

Pivot!

Working for a while in this business you get an intuitive feel for when performance matters and when it doesn’t. Of course, intuitions can be wrong, so don’t forget to measure, measure, measure. But level spawning tends to be one of these areas where performance does matter.

A level can easily have 10 000 objects or more and sometimes you want to spawn them really fast, such as when the player restarts the level. So it seems worth it to spend a little bit of time to think about how we can spawn levels fast.

This is so simple and straight forward that it might seem impossible to improve on. We are walking the resource memory linearly as we step through components, so we are being cache friendly, aren’t we?

Well, not really. We are violating one of the fundamental principles of data-oriented design: Do similar things together.

If we write out the operations we actually perform linearly instead of in an hierarchy and make things a bit more concrete, it’s easier to see:

Create entity A

Create a TransformComponent for A

Create a MeshComponent for A

Create an ActorComponent for A

Create entity B

Create a TransformComponent for B

Create a MeshComponent for B

…

Note how we are alternating between creating different kinds of components and entities. This not only messes with our instruction cache (because each component has its own code path), but with our data cache as well (because each component has its own data structures where the instances get inserted).

So let’s rewrite this so that we keep common operations together:

Create entity A

Create entity B

Create a TransformComponent for A

Create a TransformComponent for B

Create a MeshComponent for A

Create a MeshComponent for B

Create an ActorComponent for A

Much better. And we can go even further.

Instead of telling the EntityManager to “create an entity” one hundred times, let’s just tell it to “create 100 entities”. That way, if there is any economy of scale to creating more than one entity, the EntityManager can take advantage of that. And let’s do the same thing for the components:

Create entities (A, B)

Create TransformComponents for (A,B)

Create MeshComponents for (A,B)

Create an ActorComponent for A

Notice how we are encountering and making use of a whole bunch of data-oriented principles and guidelines here:

Access memory linearly.

Where there is one, there are many.

Group similar objects and operations together.

Perform operations on multiple objects at once, rather than one at a time.

For each component, we store an identifier so we know if it’s a MeshComponent, TransformComponent, etc. Then we store the number of instances of that component we are going to create and the size of the data for all those instances.

Note that now when we are walking the format, we can skip all instances of an unknown component type with a single jump, instead of having to ignore them one by one. This doesn’t matter that much, but it is interesting to note that data-oriented reorganizations often make a lot of different kinds of operations more efficient, not just the one you initially targeted.

The entity_index is used to associate components with entities. Suppose we create five entities: A, B, C, D and E and two ActorComponents. We need to know which entity each ActorComponent should belong to. We do that by simply storing the index of the entity in the entity_index. So if the entity index contained {2,3} the components would belong to C and D.

There is one thing we haven’t handled in the new layout: child entities.

But child entities are not conceptually different from any other entities. We can just add them to num_entities and add their component instances to the ComponentTypeData just as we would do for any other entity.

The only additional thing we need is some way of storing the parent-child relationship. We could store that as part of the data for the TransformComponent, or we could just store an array that specified the index of each parent’s entity (or UINT_MAX for root entities):

The compile function takes some JSON configuration data that describes the component and returns a binary BLOB of resource data for insertion into the entity resource. Note that the compile function operates on a single component at a time, because we are not that concerned with compile time performance.

When registering the compiler we specify a name, such as "mesh_component". If that name is found in the JSON data, the entity compiler will redirect the compile of the component data to this function. The name is also hashed into the component_identifier for the component.

The spawn_order is used to specify the compile order of the different component, and by extension, their spawn order as well. Some components make use of other components. For example, the MeshComponent wants to know where the enitty is, so it looks for a TransformComponent in the entity. Thus, the TransformComponent must be created before the MeshComponent.

Here the entity_lookup allows us to look up an entity index in the resource data to a an actual Entity that is created in the first step of spawning the resource. num_instances is the number of component instances that should be created and entity_index is the entity index from the ComponentTypeData that lets us lookup which entity should own the component.

So entity_lookup[entity_index[i]] gives the Entity that should own the ith component instance.

The data finally is a pointer to the instance_data from the ComponentTypeData.

That’s certainly enough for today. Next time, we’ll look at a concrete example of this.

Friday, October 3, 2014

In the last post, I talked generally about the design of components. Today I will focus on a specific real-world component, the TransformComponent.

The Transform Component

The purpose of the TransformComponent is to allow entities to be positioned in the world.

It handles positioning of entities in the world and child-parent linking. For example, you may want to link a “wheel” entity to a “car” entity, so that the wheel follows the car around when it moves.

In that sense, the TransformComponent is what forms the scene graph of an engine world.

Design Decisions

Should every entity have a transform component?

In some engines, every entity has to have a transform component, even if it is just a purely “logical” entity that doesn’t really have a position in the world.

To me it seems strange to force an entity to have a position when that has no real meaning. I also want entities to be as cheap as possible. So it seems better to have the transform component optional, just as any other component. An entity that doesn’t have a transform component doesn’t have any position in the world.

Actually, talking about the world is a bit of misnomer. The Bitsquid engine does not have a single World where everything has to live. Instead you can create multiple worlds, each populated with its own objects. So you might have one world for your “main game”, one world for the “inventory screen”, one world for the “loading screen”, etc.

This is a lot better than having an “inventory room” at some secret hidden place in the main game world.

Each world has its own TransformComponent manager, and an entity can create transform components in several of these managers, if it so desires. So the same entity can exist and be positioned at different places in different game worlds. Since a MeshComponent manager also exists in each world, the entity can have different graphical representations in each world.

This is a bit esoteric, and I don’t expect many entities to make use of this, but there are situations when it could be interesting. For example, a player’s pickaxe could exist both in the “game world” and in the “inventory world” and still be managed as the same entity.

Entity scene graphs and model scene graphs

In the entity system there are really two kinds of “scene graphs” that we need to deal with.

The first is the one we have already talked about, the graph formed by entities and their linked child entities.

The second is the graph of nodes within an entity. For example, a character entity may have a model with hundreds of bones that can be individually animated.

What should the relationship be between these two graphs?

In previous engine code, I have always treated these two graphs as parts of the same system. The model scene graphs were linked to nodes in the entity scene graphs and computed their transforms in world space. This creates an update order dependency. We can’t compute the world positions in the model scene graph until we have computed the world position in the entity scene graph. This limits what kinds of things we can do in parallel.

For the entity system I’ve decided to decouple these two concepts. The model scene graph won’t compute world space poses, instead it will compute poses relative to the entity pose. This means that we can evaluate the animations and compute the model pose without knowing anything about the entity pose. (Ignoring world space constraints, of course, but they will be handled in a later pass.)

Of course it also requires us to multiply the model node transforms with the entity transform to get the actual world position of the model nodes.

I have not completed the design of the model scene graph component yet, but maybe I’ll get a chance to return to this in a future post.

Immediate or deferred updates

In previous engines I have always used deferred updates of the world transforms. I.e., changing the local transform of a node would not immediately update its world transform (or the world transforms of its children). Instead it would simply set a “dirty” flag in the entity. Later, I would compute the world transforms of all the dirty nodes (and their children) as a single step.

This has the advantage that we never have to compute the world transform of a node more than once.

Consider the worst case scenario, a long chain of nodes:

[ node_1 ] ---> [ node_2 ] ---> [ node_3 ] ---> ... ---> [ node_n ]

With a deferred update, changing the local pose of every node will still just require O(n) computations to compute all the world transforms. With an immediate update, where we compute the world transforms of all children as soon as the parent transform changes, we will need O(n^2) computations.

On the other hand, there is a drawback to using deferred updates. Whenever we ask for an object’s world position we won’t get its actual world position, but its world position from the last frame (unless we ask after the world transform update). This can lead to a lot of confusion and subtle bugs. Solving them often requires ugly hacks, such as forcing graph updates at different times.

So what should we choose?

I think that with the decision to decouple the model scene graphs from the entity scene graphs the performance problems of immediate updates are a lot less serious. Long chains of nodes that are all moving can certainly exist in the model scene graph. (Consider an animation of a character swinging a whip.) But I would guess that long chains of objects that are all moving at once are a lot less common in the entity scene graph.

Note that the performance problems do not appear if it is just the root entity that is moving. In that case, both the immediate and the deferred update will be O(n). It is only when the parent and the children are moving that the immediate update does worse.

I don’t expect there to be very long chains of entities (n <= 5 ???) and I don’t expect all of the objects in those chains to be moving simultaneously. So I have decided to go with immediate updates so that we always have accurate world transforms.

Note: If we run into performance problems as a result of this, we can always create an API function that allows us to set multiple local transforms at once while doing a single world transform update, thus getting back the O(n) performance.

A side note on deferred updates

Note that if you want to do deferred updates, you want to keep the entity array sorted so that parents always appear before children. That way you can just walk the array from beginning to end and compute the transforms and be sure that the world transform of a parent has been computed before you compute the world transform of its children.

Also, you don’t want to loop over the entire array to look for dirty objects:

for (int i=0; i<n; ++i) {
if (dirty[i])
transform(i);
}

Typically, in a scene, only a small percentage of the objects are moving at any one time (maybe as little as 1 %). So looping over all objects, even just to check a flag, can waste a lot of time.

A better solution is to sort all the dirty objects to the end of the array, so we can loop over just them:

for (int i=first_dirty; i<n; ++i)
transform(i);

Since we only need a partial sorting of the array, we don’t have to run an expensive O(n log n) sorting algorithm. (It would kind of defeat the purpose to run an O(n log n) sort to avoid an O(n) update.) Instead, we can achieve this by judicious swapping.

When a node becomes dirty we move it to the start of the dirty list by swapping it with the element before the dirty list and decreasing first_dirty:

This guarantees that parents are always processed before their children.

We also need a way to move items off the dirty list, or it will continue to grow indefinitely. We could clear the list every frame, but that might lead to a lot of swapping as items are moved in and out of the list. A better approach might be to check if an item hasn’t moved in five frames or so, and in that case we move it off the dirty list. This avoids swapping those items which are always moving.

When using the immediate update strategy, sorting the list is not as important, but we can employ similar swapping strategies to make sure that a parent node and its children are kept close together in the array, so that the immediate update is cache friendly.

Implementation

With the design thought through, there is really not that much to the implementation.

Just as in the last post, we store the transform component data for all instances in a single big memory block:

The parent, first_child, next_sibling and prev_sibling arrays all store instance indexes. We can find all the children of a particular entity by following the first_child link and then the next_sibling links of that link.

Note: I’ve written this as a recursive function for easier reading, but you might want to rewrite it as an iterative function for better performance.

Note that when you swap two instances in the array (to do swap-erase or to sort the array as described above), in addition to swapping the entries in the array you also need to take care to keep all the parent, first_child, next_sibling and prev_sibling references intact. This can get a little hairy, especially when you are changing references and trying to walk those lists of references at the same time. My suggestion when you want to swap two instances [A] and [B] is to use the element at the end of the array [size] as a temporary storage slot and instead of trying to do everything at once, use three steps:

// Move element at A (and references to it) to size.
[size] <--- [A]
// Now nothing refers to A, so we can safely move element at B (and references
// to it) to A.
[A] <--- [B]
// And finally move the element at size to B.
[B] <-- [size]